1. Field of the Invention
The present invention relates to methods for removing insulating films such as native silicon oxide films formed on a surface of a silicon wafer, a surface of a polysilicon film, a surface of an amorphous silicon film and the like (hereinafter the "silicon layer "surfaces"). Furthermore the present invention relates to a method for selectively removing insulating films such as thermal silicon dioxide film (th-SiO.sub.2), a silicon nitride (SiN.sub.2) film a phosphosilicate glass (PSG) film, a boro-phospho silicate glass (BPSG) film or an arsenosilicate glass (AsSG) film forming a semiconductor device, in addition to the native silicon oxide film.
The above-mentioned PSG film is a binary oxide of P.sub.2 O.sub.2, and SiO.sub.2 and generally is deposited in the SiH.sub.4 +O.sub.2 +PH.sub.3 gas system by CVD (Chemical Vapor Deposition) method, while BPSG film is a ternary oxide of B.sub.2 O.sub.3 -P.sub.2 O.sub.5 -SiO.sub.2 and generally deposited in the SiH.sub.4 +O.sub.2 +PH.sub.3 +B.sub.2 H.sub.6 gas system by the CVD method.
2. Description of the Related Art
In a process of manufacturing a semiconductor device, contamination sometimes occurs and which adversely affects the operation characteristics of the device. One of the contaminants is a native silicon oxide film formed on a silicon layer surface.
A native oxide film is easily formed to a thickness of 10 to 20 .ANG. on a surface of a silicon layer by merely exposing the same to be atmosphere. A native oxide film is also formed secondarily in a cleaning or an etching step in a semiconductor device manufacturing process.
It is well known that the electric characteristics of, for example, a thin gate oxide film, are considerably affected by the pretreatment of a silicon wafer. Consequently, in order to form a thin oxide film such as a gate oxide film on a silicon wafer in a semiconductor device manufacturing process, it is necessary to remove the native oxide film in advance from the surface of the silicon wafer.
It is also well known that normal electrode function cannot be obtained if the native oxide film remains on the surface of a silicon wafer where electrodes such as a source, a drain and the like are to be formed. In forming a metallic electrode, the native oxide film should be completely removed from a silicon layer surface in order to suppress contact resistance. Also during epitaxial growing of silicon, it is necessary to remove in advance the native oxide film from the silicon layer surface.
Thus, the native silicon oxide film formed on the silicon layer surface must be removed in a semiconductor device manufacturing process, particularly prior to forming a film by chemical vapor deposition (CVD), sputtering or the like.
Methods using hydrogen fluoride (HF) gas have been recently studied for removing native silicon oxide films from silicon layer surfaces. For instance, Japanese National Publication No. 62-502930 of International Patent Application (PCT/US86/01508) proposes the method described below. According to this proposed method, thin hydrogen fluoride gas is supplied together with water vapor (H.sub.2 O) to a surface of a silicon wafer. The silicon wafer surface is exposed to the gas and vapor. As a result, various silicon oxide films can be removed in an atmosphere of high humidity.
It is to be noted that in manufacturing a semiconductor device, a surface of a silicon wafer is subjected to various film forming steps. Not only a native oxide film but also silicon insulating films such as a silicon oxide film, a silicon nitride film, a PSG film, a BPSG film and an AsSG film are formed by thermal oxidation, CVD and other methods on the silicon wafer. According to the method disclosed in the above-mentioned publication, not only the native oxide film but also the silicon insulating film being formed especially on the wafer are removed from the wafer surface.
On a silicon wafer and also on a semiconductor wafer such as a gallium arsenide (those wafers being generally referred to hereinafter as the "wafers"), there may be formed any film on a polysilicon film or an amorphous silicon film previously formed on the wafer. In this case also, it is necessary to remove in advance the native silicon oxide film on the surface of the polysilicon film or amorphous silicon film.
In view of the above, methods for selectively removing native oxide films have been proposed, for instance, in "Special Issue No. 2 of Nikkei Micro Device" (Nikkei McGraw-Hill, October 1988, pp. 202-207), or in "Submicron ULSI Process Technology" (Papers Prepared for Ultra LSI Ultraclean Technology Symposium No. 7, published by Realize Company, July 1988, pp. 173-193). Those methods utilize the fact that there is a boundary concentration of HF and H.sub.2 O components, above which a silicon oxide film is etched and below which no etching occurs, and the fact that there is a difference in boundary concentration between a native silicon oxide film and a thermal silicon oxide film or the like.
According to those methods, the HF gas concentration in nitrogen (N.sub.2) gas for dilution is controlled under a condition of a considerably low concentration of H.sub.2 O component in the atmosphere. This makes it possible to selectively remove only the native oxide film from the silicon wafer surface.
However, according to the above-described methods, it is necessary to accurately dilute HF gas by N.sub.2 gas and to generate diluted HF gas of a concentration of several percentage points by volume. In addition, it is necessary to extremely decrease the levels of H.sub.2 O in the atmosphere. Thus, the structure of the entire apparatus becomes complicated and it is not easy to control the apparatus.